1. Field of the Invention
The present invention relates to and apparatus and method for measuring a patient""s airway or anatomical deadspace volume non-invasively based on a quantitative measurement of the flow of gas from the patient and a measurement of a constituent of that gas.
2. Description of the Related Art
Medical ventilators are utilized to ventilate a patient by engendering the exchange of gas in the lungs of the patient. It is a goal of medical ventilation, for example, to reduce as much as possible the patient""s physiological deadspace VDPHY, which is the total amount of volume in the patient where no exchange of oxygen and carbon dioxide occurs. The patient""s physiological deadspace volume VDPHY includes (1) the anatomical deadspace volume VDANA, which is the volume of the patient""s conducting airway, e.g., the airway from the nose and/or mouth and the alveoli in the lungs, (2) the alveolar deadspace volume VDALV, which is the volume of the lungs where, even during normal unassisted breathing, no exchange of oxygen and carbon dioxide occurs and (3) the relative deadspace volume VDREL, which is the volume of the lungs were some exchange of oxygen and carbon dioxide takes place, but the amount of exchange is below that of a normal lung.
An experienced caregiver can estimate to a medically reasonable degree of accuracy a patient""s total lung volume or tidal volume VT, which includes the alveolar volume VA, where carbon dioxide and oxygen are exchanged, and the alveolar deadspace volume VDALV and relative deadspace volume VDREL. In contrast to the estimate of the patient""s total lung volume, however, an estimate of the patient""s anatomical deadspace volume VDANA is not as accurate. Knowing the patient""s anatomical deadspace volume is important because an underestimation of the patient""s anatomical deadspace volume can result in the ventilator supplying an insufficient volume of breathing gas to fill the total lung volume of the patient during inhalation. Conversely, an overestimation of the anatomical deadspace volume can result in the ventilator attempting to overfill the patient""s lungs, with corresponding patient discomfort and increased risk of pulmonary trauma.
Accordingly, it is an object of the present invention to provide an apparatus for measuring a patient""s anatomical deadspace volume VDANA that overcomes the shortcomings of conventional measurement/estimation techniques. More specifically, it is an object of the present invention to provide an apparatus that accurately, non-invasively, and repeatedly determines the anatomical deadspace volume of a patient.
This object is achieved according to one embodiment of the present invention by providing an anatomical deadspace measurement apparatus that includes a sensor adapted to measure a parameter indicative of a volume of gas exhaled by a patient during at least an exhalation phase of a respiratory cycle. The apparatus also includes a gas analyzer, such as a capnometer or oxygen analyzer, that measures a concentration of a gas constituent, such as carbon dioxide (CO2) or oxygen (O2), in the patient""s expiratory flow. The apparatus further includes a controller that receives the outputs of the sensor and gas analyzer and determines the patient""s anatomical deadspace volume based on these outputs. In an exemplary embodiment of the present invention, the controller determines the anatomical deadspace volume of a patient by determining a time t1 that corresponds to a point at which the patient commences exhaling and a time t2 that corresponds to an inflection point in a waveform corresponding to the concentration of the gas constituent measured by the gas analyzer. The controller calculates the volume of gas exhaled by the patient from time t1 to time t2 as the anatomical deadspace volume of the patient.
It is yet another object of the present invention to provide a method of determining an anatomical deadspace volume VDANA of a patient that does not suffer from the disadvantages associated with conventional measurement/estimation techniques. This object is achieved by providing a method that includes: (1) detecting a parameter indicative of a volume of gas exhaled by a patient, (2) detecting a concentration of a gas constituent in the patient""s expiratory gas flow, and (3) determining the patient""s anatomical deadspace volume VDANA based on the detected volumetric flow of gas and the detected concentration of a gas constituent. In an exemplary embodiment of the present invention, the last step, i.e., step (3), includes determining a time t1 that corresponds to a point at which the patient commences exhaling, determining a time t2 that corresponds to an inflection point in a waveform corresponding to the concentration of the gas constituent measured in step (2), and calculating a volume of gas exhaled by the patient from time t1 to time t2 as the anatomical deadspace volume of the patient.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.